This invention relates to a process for plating through-holes of a printed circuit board, more particularly to a process for pretreatment before plating the through-holes of a printed circuit board.
Printed circuit boards are provided with through-holes which connect one circuit to other circuits. Generally, a process for pretreatment before plating printed circuit board through-holes is predominantly composed of the steps of degreasing, washing, coarsening of copper film surfaces, washing, dipping in a sulphuric acid solution, washing, catalyzing, washing, accelerating and washing. After this pretreatment, electroless copper plating is carried out onto the through-holes. No step of removing air within the through-holes is included in the above-mentioned pretreatment process, but a surface active agent contained in a degreasing liquid used in the degreasing step reduces the surface tension of the liquid to improve the wettability of the liquid to the circuit boards. Furthermore, raising of a liquid temperature is also effective to some extent for removing air within the through-holes. In the case where a rack is used to convey and dip the circuit boards into a degreasing liquid in a greasing bath and the circuit boards on the rack are slightly impacted when the rack enters a rack receiver at the upper end of the bath, air may be removed from the through-holes. Therefore, any intended removal of air within through-holes of circuit boards having an aspect ratio (board thickness/through-hole diameter) of about 5 or less and hole diameter of 0.5 mm or more, is not carried out in the state of art.
However, printed circuits have recently been more highly densified, so such printed circuit boards have a smaller diameter of through-holes. In other words, the circuit boards tend to have a greater aspect ratio. In such case, air remaining within through-holes is difficult to remove completely. When electronic parts are mounted on the printed circuit boards and soldered, blow holes may be generated. The generation of the blow holes is mainly caused by gas ejected from the printed circuit boards to cause micro voids in the through-holes of the boards, which voids are not plated. Furthermore, there are various causes for the generation of the micro voids, but the most important cause is considered that air firmly adhered to the inner walls of the through-holes is not completely removed. The micro void-generating rate is about 0.01% or less per the number of through-holes. When one circuit board has as a great number of through-holes as about 35,000, the number of rejected circuit boards produced cannot be disregarded by any means.
Furthermore, circuit boards having an aspect ratio of about 5 or more and a hole diameter of about 0.5 mm or less have an undesirable area on which no copper is precipitated in a ring form substantially at the middle portion of a through-hole. This is referred to as a ring void. In case of using a chromium ion-containing sensitizing agent, the generation of the ring voids is caused by insufficient washing or using of improper compositions of the sensitizing agents or improper sensitizing conditions. The ring voids generated even when the proper compositions of sensitizing agent and proper sensitizing conditions are used are considered to be attributed to air remaining within the through-holes. That is, an increased aspect ratio causes a greater amount of air to remain in a form of bubbles within the through-holes, especially at the middle portion of the holes. Therefore, it is considered that the inner walls of the through-holes brought into contact with such air bubbles are not pretreated before plating, and accordingly copper is not precipitated on the such inner walls of the holes.
In order to avoid the generation of the ring voids, Japanese Patent KOKAI (Laid-Open) Nos. 46781/78 and 46782/78 proposed that circuit boards be subjected to low-frequency vibration or mechanical shock action. For example, they showed that the ring void-generation rate was about 5.times.10.sup.-3 % for a circuit board having a thickness of 3.8 mm and a hole diameter of 0.4 mm, when pretreated in an ordinary pretreatment manner but, on the other hand, the ring void-generation rate was reduced to 2.times.10.sup.-5 % when the same board was subjected to the proposed vibration or shock action. However, a great number of rejected boards were produced, where the board had a great number of through-holes. Thus, the proposal of the above-mentioned Japanese Patent KOKAI is not satisfactory.